1. Field of the Invention
This invention relates to a radiation image information read out apparatus for scanning a stimulable phosphor plate with a light beam of stimulating rays to cause the stimulable phosphor plate carrying radiation image information to emit light according to the radiation image information stored therein so as to read out the information, and more particularly to a device for controlling the gain of the read out system according to various conditions.
2. Description of the Prior Art
A stimulable phosphor stores a part of the energy of a radiation when exposed to the radiation like X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays and ultraviolet rays. Then, when the stimulable phosphor which has been exposed to the radiation is exposed to stimulating rays, light is emitted from the stimulable phosphor upon stimulation thereof according to the stored energy of the radiation.
An X-ray image forming system has been proposed, in which an X-ray image of a human body, etc. is once recorded in a sheet-like stimulable phosphor plate, the stimulable phosphor plate is then scanned with stimulating rays to cause the stimulable phosphor to emit light upon stimulation thereof, the light emitted is detected by a photodetector to obtain an image signal, and the image signal is used to modulate the light beam for recording the X-ray image on a recording medium such as photographic film. Such a system is disclosed, for example, in U.S. Pat. Nos. 4,258,264 and 4,239,968.
With such a conventional system, however, when the stimulable phosphor plate storing the radiation image information is scanned with a laser beam to read out the light emitted therefrom upon stimulation thereof, the intensity of light detected fluctuates largely even if the laser beam having a constant intensity is used for scanning. Thus, such a conventional system cannot provide a radiation image which is easy to observe. This is because the radiographic conditions such as exposure dose, quality of radiation and object during radiography are not always the same. If the gain of the read out system for the radiation image, such as photodetector, is fixed without being adjusted according to the radiographic conditions, the following problem arises. That is, the gain of the read out system may be adequate when reading some pieces of radiation image information. However, when reading other pieces of radiation image information, the gain will be too low and the radiation image information cannot be read out correctly due to noise.
In addition, the output of the laser source used to emit stimulating rays is unstable and the intensity thereof inevitably fluctuates about 10 to 20% with time. This fluctuation in the intensity of the stimulating rays causes the level of the light emitted from the stimulable phosphor upon stimulation thereof to fluctuate. This is because the intensity of the light emitted from the stimulable phosphor is approximately proportional to that of the stimulating rays. Thus, sections in one image, which should have the same density, are reproduced to different densities. Therefore, it is not possible to reproduce correct radiation image information.
Accordingly, in order to always achieve correct read out of the radiation image information stored in the stimulable phosphor, it is necessary to control the read out gain so that the image signal from the light detecting system is within a constant range even when the radiographic conditions such as exposure dose, quality of radiation and object are different and when the intensity of the stimulating ray source changes.